APPLICATION OF A GEOCHEMICAL MODEL TO UNDERSTAND ARSENIC INTERACTIONS WITH IRON OXYHYDROXIDES IN A CRYSTALLINE BEDROCK AQUIFER

A geochemical model was utilized to help understand the interactions between dissolved arsenic in groundwater and iron oxyhydroxide coated crystalline bedrock aquifer materials. Water samples from 127 bedrock wells in the study area had arsenic concentrations ranging from 26 nmol/L to 5300 nmol/L, with a median concentration of 210 nmol/L. Approximately 62% of all homes sampled in this arsenic affected area exceeded the regulatory limit of 130 nmol (10 µg/L). All samples contain considerably more sulfate that iron on a molar basis. The primary source for sulfate in this system is sulfide oxidation, suggesting that high sulfate/iron ratios are due to removal of iron from solution, rather than addition of sulfate from another source. Water samples plot just inside the stability field for Fe²⁺ for the corresponding measured Eh and pH conditions for that water sample. Calculated iron oxyhydroxide formation rates for this groundwater system range from 0.2 µmol/L/year for low pH samples up to 200 mmol/L/year for high pH samples. The chemical evidence of iron oxyhydroxide precipitation is supported by field observations of iron staining along fractures in most outcrops sampled during the course of the study.

Arsenic concentrations in solution range from 70 nmol/L at a pH of 5.6 up to 4.23 µmol/L at a pH of 8.25. The relation between arsenic adsorption to iron oxyhydroxides at various pH values and solution chemistries was modeled using the software program Geochemist’s Workbench. Model calculations were conducted at a concentration of 1000, 100, 50 and 25 mg HFO/kg H₂O. This range of values for HFO approximately equates to the precipitation of all the iron (~2-5 mg/kg) from the samples with the highest iron concentration over 3 to 300 pore volumes in a fracture. For 1000 mg/kg HFO, arsenic concentrations in solution are extremely low from pH 5 to pH 7, with nearly 100% of the arsenic adsorbed to the HFO. At pH values approaching 8, arsenic concentration in solution increases markedly up to a value of 4.23 µmol/L, which requires all of the arsenic in the model system to be in solution. Use of the geochemical model helped understand the precise nature of arsenic adsorption and supports the hypothesis that arsenic concentrations in this aquifer are strongly controlled by adsorption to iron oxyhydroxides.